Vulkan: Don't crash on diagnostics with null pMessageIdName This can be null and this will causes a crash in strstr(). Check for null before dereferencing this field. BUG=angleproject:2475 TEST=angle_end2end_tests on Fuchsia and angle_enable_vulkan_validation_layers=true Change-Id: I76a2738365838cb2f9cb54af7637417735762b69 Reviewed-on: https://chromium-review.googlesource.com/c/1446506 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>